1. Field of the Invention
This invention relates to a ceramic article and a method for the production of the article, and more particularly to an article and method comprising alternating layers of dissimilar materials to produce an article with an improved work of fracture.
2. Description of the Prior Art
Ceramic articles are, of course, well known and find many commercial uses where, for example, hardness, refractory properties or relative chemical inertness are desired. A serious deficiency of ceramic products, however, is their brittleness or, stated in other words, their poor work of fracture or toughness. This limitation has hindered the entry of ceramics into those areas where their other properties would be highly desirable, for example, U.S. Pat. Nos. 5,657,729 and 5,687,787 describe attempts to incorporate toughened ceramic parts into internal combustion engines.
Brittle materials typically fail catastrophically and often without warning. Conversely, tough materials will normally bend or deform before failure. In most applications, the latter type of failure is preferred. Common methods of testing toughness are a Single Edge Notch Bend (SENB) test and a Modulus of Rupture (MOR) test. Both involve a three point bending geometry and differ in the presence or absence, respectively, of a notch in the sample to be tested. In both, a stress on a sample is slowly increased as a function of strain. The resultant area beneath a plot of stress versus strain is the work of fracture and represents the amount of energy absorbed during one of these tests.
A tougher material has the ability to absorb greater amounts of energy than a more brittle material. One way a material may absorb energy is by microscopic morphological changes. For example, tough metals or alloys like steel absorb energy by, for example, developing dislocations, slipping across crystal planes, or undergoing crystal twinning. A material may also absorb energy by creating new surface area through a process known as crack blunting. For example, composite materials, such as fibreglass, are heterogeneous and contain a plurality of phases. When a crack reaches a phase boundary, the crack may propagate along the boundary, and create a delamination crack. In effect, the crack is blunted at the phase boundary. Blunting reduces crack propagation by spreading the energy at the crack tip over a larger area.
Generally, ceramic materials cannot absorb much energy because their crystal structure resists microscopic morphological changes. Additionally, crack blunting does not occur to any substantial extent in homogeneous materials. Attempts to improve the toughness of ceramics have concentrated on introducing some degree of heterogeneity into the ceramic. For example, an increase in toughness has been accomplished by providing a second phase within the ceramic, such as a layer of fibers, see, e.g., U.S. Pat. No. 5,589,115. Presumably, the fiber layer disrupts crack propagation by blunting the crack tip. Unfortunately, this solution is not without its flaws. The green ceramic matrix, in which the fiber is placed, shrinks when fired, but the fiber itself does not. This results in delamination of the fiber from the ceramic and creates what are essentially voids in the brittle ceramic. Voids normally act to concentrate stresses, initiate crack formation, and increase the likelihood of brittle failure. Techniques to overcome this problem involve a plurality of mats of ceramic fibers impregnated with a particulate ceramic material, liquid diluent and organic binder. This technique places the ceramic particulate in more intimate contact with the fiber. During firing, however, the ceramic particulate still shrinks. While an improvement over the prior art, this method does not completely overcome the delamination problem, and results in a ceramic composition with variable mechanical properties.
Delamination can be substantially overcome by a technique involving melt infiltration. This technique involves perfusing a molten ceramic material into ceramic fibers. Although delamination is reduced, several new problems arise. Very high temperatures are required to melt ceramics and some ceramics sublime before they melt. The high temperatures can also damage the ceramic fiber. Even if the ceramic can be melted, the viscosity of a molten ceramic is so high that the rate of infiltration into the fibers is very slow and the molten ceramic may not homogeneously wet the surface of the fibers.
The extremely high temperatures of melt infiltration can be avoided by a vapor infiltration technique, see, e.g., U.S. Pat. No. 5,488,017. At relatively low temperatures, a vapor comprising a ceramic precursor infiltrates ceramic fibers. Later the chemical is decomposed to leave a ceramic residue. For example, gaseous methyltrichlorosilane may be deposited onto ceramic fiber at just several hundred degrees centigrade and later decomposed to silicon carbide at a temperature which may be less than 1200xc2x0 C. A silicon carbide matrix is created which is reinforced by the ceramic fiber. Although overcoming some of the disadvantages of previous processes, vapor infiltration is very time-consuming and limited to ceramics with volatile precursors.
U.S. Pat. No. 5,591,287 avoids using fibers, melts or volatile precursors. This patent creates one or more zones of weakness between layers of sinterable, particulate ceramic material. The zones of weakness consist of very thin layers of non-sinterable or weakly sinterable material. Examples of a non-sinterable material include carbon or an organic polymeric material, which may pyrolyze into carbon. A weakly sinterable material may form bonds with itself and the sinterable, particulate ceramics, but the bonds so formed should be substantially weaker than the bonds formed within and between the sinterable ceramic layers.
The zones of weakness should be less than about 50 microns to permit sintering between ceramic layers. Such thin zones of weakness may be created by spreading a suspension of non-sinterable or weakly sinterable material over one surface of a preformed, sinterable ceramic. Many zones of weakness may be produced by depositing the non-sinterable material between each of a plurality of ceramic layers. The resulting zones of weakness may deflect cracks propagating through the ceramic. The crack may then travel along the zone of weakness and form a delamination crack between the layers of ceramic. The process of delamination increases the work of fracture. Unfortunately, this method is limited to sinterable ceramic materials that have been preformed into a layer over which a non-sinterable material can be spread. This restricts both the composition and the geometry of articles, which may be made using this method.
Despite these known methods for improving the toughness of ceramic articles, there is still a need in the industry for a method to produce quickly and cheaply a tough morphology in a commercially useful shape. Simply mixing a ceramic fiber into a sinterable ceramic often leads to delamination between the two materials. Methods to prevent delaminations are either too time-consuming, limit article geometry or composition, produce inconsistent results, or require excessive temperatures. A commercially viable method is needed to toughen a ceramic article.
The present invention relates to a multilayer ceramic article and a method of making the same. In a broad aspect, the article comprises a plurality of layers of a first phase comprising a fused and/or carbon bonded particulate ceramic; and, disposed between adjacent layers of first phase a layer of a mechanically or chemically different second phase. The article of the present invention is depicted as possessing a substantially improved work of fracture compared to a ceramic article without a layered structure.
The first phase is described as a fused or carbon-bonded, particulate ceramic. The second phase may be a porous material, such as a metal mesh, or a weakly fused or carbon-bonded refractory, or may even comprise the pyrolyzed residue of a combustible material. Alternatively, the second phase may be fused by a process independent of the first phase, such as by sintering if the first phase is a carbon-bonded ceramic. In other embodiments, the second phase may share a similar bonding mechanism with the first phase but will possess significantly weaker mechanical properties.
The invention describes layers of the first phase as preferentially having a thickness from between about 0.5 mm to about 10 mm with layers of the second phase having a thickness from about 0.005 mm to about 2 mm.
One aspect of the invention describes the layers as spiralling along the longitudinal axis of a cylindrical shape. The article may also comprise a bore. The present invention also relates to a method for producing a ceramic article having improved thermal shock-resistance and toughness. In a broad aspect, the invention describes a method to fashion a ceramic article by alternating layers of a first material with a second material. The first material may be a fusible or carbon-bonded, particulate ceramic. The second material is expected to form a weakly fused or weakly carbon-bonded layer. Alternatively, the second material may fuse by way of a process independent of the first material, such as by sintering if the first material is a carbon-bonded ceramic. The second material may be proffered as a sheet, film, membrane, or even a casing onto or into which the first material may be placed. The layers are then pressed into a piece and fired to form the finished article.
In one aspect of the invention, the second material is described as a combustible material, which may pyrolyze at elevated temperatures. The combustible material may be an organic material, such as plastic, paper, cotton or other natural or synthetic polymer.
Still another aspect of the invention describes a process to make a layered, cylindrical article. The first material is described as a ceramic refractory and the second material may be a combustible sheet. Layers are alternated by coating the combustible sheet with the ceramic, compacting the ceramic on the sheet, and subsequently rolling the coated sheet onto itself thereby creating a cylinder with a xe2x80x9cjelly rollxe2x80x9d morphology. The second material may alternatively be a tubular casing. The ceramic material may then be inserted into the casing, compacted, and formed into any desired shape, including a xe2x80x9cjelly roll.xe2x80x9d
A still further aspect of the invention describes a method of making the article into a tube by wrapping a coated sheet or filled casing around a mandrel, pressing the wrapped sheet or casing on the mandrel, removing the piece from the mandrel, whereby a bore is created where the mandrel had been, and firing the wrapped sheet or casing to make the article.
Other details, objects and advantages of the invention will become apparent as the following description of a present preferred method of practising the invention proceeds.